1. Field of the Invention
This invention relates to an image forming apparatus having a fixing unit.
2. Description of Related Art
Prior art image forming apparatuses such as electrophotographic printers heat a heating roller by generating heat at the heater equipped inside the heat roller, and fix the transferred developer image onto a medium such as recording paper using the heat. Such an image forming apparatus having the fixing unit generally includes a temperature control unit to keep the surface temperature of the heat roller at a prescribed temperature. Such a temperature control unit controls the heater based on detected results of the surface temperature with a proper temperature detection unit such as, e.g., thermistor and keeps the surface temperature of the heat roller at the prescribed temperature. Japanese Unexamined patent publication Heisei No. 11-327350 discloses such an art.
In this prior art, the temperature control unit makes the heater generate heats by turning on power supply to the heater where the temperature detection unit detects that the surface temperature of the heat roller is at a fixable temperature or below. The temperature control unit halts heat generation with the heater by turning off the power supply to the heater where the temperature detection unit detects that the surface temperature of the heat roller is at a fixable temperature or above.
Referring to FIG. 17, the prior art temperature control unit operates a feedback control entering heater #1 signal at a time when the temperature detection unit detects the lowered surface temperature of the heat roller constituting the fixing unit, or namely, at timings B1, B3, B5.
As shown in FIG. 17, where continuing printing is operated at the prior art image forming apparatus, however, the surface temperature of a backup roller is increased because the heat retained at the heat roller is transmitted to the backup roller by direct contacting the heat roller heated by the heater with the backup roller until an Nth page of the recording paper reaches the nipping portion after the N−1 page of the recording paper passes by the nipping portion formed with the contacting surface between the heat roller and the backup roller. The heat retained at the heat roller and the backup roller dissipates to the recording paper where the first half of the Nth page of the recording paper passes through the nipping portion, so that the surface temperature of the heat roller and the backup roller decreases. Therefore, this operation surely secures the first half of the Nth page of the recording paper, but brings a result that the second half of the Nth page of the recording paper is not fixed adequately. In addition, the second half of the Nth page of the recording paper is still not fixed well even where the temperature control unit outputs the heat #1 signal at the timings B1, B3, B5, because time lag exists before the heat roller is actually heated.
It is an object of the invention to provide an image forming apparatus reducing occurrences of impaired fixing of developer images at the second half of the recording paper during a fixing step done by a fixing unit.